Voltage drop

mmm. but the propagation velocity depends on the dielectric constant so it's slower for 2.5mm twin and earth.

Martin

Yes, the propagation velocity will be about 60% of that in free space so the wavelength at 50 Hz will be about 10,000 km.

Russell

I'm afraid that is incorrect. The cable length will be shorter by the velocity factor i.e.approximately 3600km.

Regards

Brian (Retired Telecommunications Professional)

Edited By Brian Oldford on 24/02/2017 17:45:53

Haha! Why am I thinking of Cerenkov radiation? Anyway, coaxial stubs, filters, Hi-Q breaks, on 144 & 432 MHz

Geoff

v=fλ

So decreasing velocity decreases wavelength.

Neil

Or even an odd integer 1/4 wave length of 75 ohm phasing harnesses for a pair of stacked yagis.

High current interconnectors use DC to transmit power over long distance then covert it back to AC to reduce losses.

No, Neil, in a given medium, 'free space', open wire line, coaxial cable etc., the velocity is the same for all frequencies. In a comparison between mediums, the velocity may be different, one to another, and a resonant or tuned length will be longer or shorter by that factor.

Geoff

The speed of an electromamgnetic wave (so radio, light, x rays, gamma rays etc) in vacuum is approximately 300,000km/sec.

In a cable the speed is then dependant on the velocity factor, which is a ratio of the speed of the wave in the cable related to that in vaccuum. In most typical cables or other electromagnetic media (fibreoptic cable, lenses, etc) the speed is proportional to the square root of the dielectric constant of the material (also the permeability for case of magnetic materials). So most cables have insulation of plastic (polythene, PTFE, etc) having dielectric constant around 2 to 2.5, so the velocity factor is around typically 70% for solid dielectric, or maybe 75% for air-spaced plastic dielectric.

The velocity factor also simply the inverse of the refractive index of the material: so for a dielectric constant of say 2.25, the refractive index is 1.5, and the velocity factor is 0.667. (Assuming the material is non-magnetic).

So since the speed of light is 300km/sec or 186,000 mile/sec: if you connect yourself to a mile of cable and then have high voltage power applied at the other end, it will take 5.3 microseconds before you are electrocuted for open wires, or 7 microseconds through typical plastic-insulated cable.

If the the cable were 70 light-years long, you then need not worry

Martin

Edited By Martin Whittle on 24/02/2017 23:06:42

Edited By Martin Whittle on 24/02/2017 23:07:34

Edited By Martin Whittle on 24/02/2017 23:20:06

Embarrasing error for a retired electronic engineer. Must have had too much wine with my lunch

Russell

Will I have to check the speed of light before I can use my extension lead? It's near 50 metres long with rubber outer.

yes ,and the back e.m.f ionisation differential.